1. Field of the Invention
The present invention relates to a receiving apparatus.
2. Description of the Related Art
In a transition period from analog broadcasting to digital broadcasting of radio or television, simultaneous broadcasting may be executed in which the same program is broadcasted both in the analog broadcasting and the digital broadcasting. For example, in radio broadcasting by the IBOC (In-Band On-Channel) method, as illustrated in FIG. 8, a broadcast wave of the digital broadcasting (hereinafter referred to as a digital broadcast wave) whose contents is the same as those of the analog broadcasting is transmitted using both sides of the broadcast wave of the analog broadcasting (hereinafter referred to as an analog broadcast wave). Thus, a receiving apparatus supporting the digital broadcasting (hereinafter referred to as a digital receiving apparatus) can receive the digital broadcast wave and reproduce high-quality sound, while even a receiving apparatus supporting the analog broadcasting (hereinafter referred to as an analog receiving apparatus) can still receive the analog broadcast wave. Therefore, it is possible to smoothly spread the use of the digital receiving apparatuses and shift from the analog broadcasting to the digital broadcasting, while keeping reception by the analog receiving apparatus.
Also, when the simultaneous broadcasting is executed as above, it is also possible to receive both the analog broadcast wave and the digital broadcast wave, and reproduce high-quality digital broadcasting sound (hereinafter referred to as digital sound) and existing analog broadcasting sound (hereinafter referred to as analog sound) with switching them according to a reception state. For example, in Japanese Patent Laid-Open Publication No. 2006-115200, a receiving apparatus (receiver) is disclosed that is capable of adjusting a time difference or a volume difference between the analog sound and the digital sound, to decrease a sense of discomfort at switching of the sounds.
Here, in the radio broadcasting by the IBOC method, an example of a configuration of the receiving apparatus is illustrated in FIG. 7, which is capable of adjusting the time difference and the volume difference between the analog sound and the digital sound as in the case of the receiving apparatus disclosed in Japanese Patent Laid-Open Publication No. 2006-115200. In the receiving apparatus illustrated in FIG. 7, the analog broadcast wave is converted into an audio signal through an antenna 1, a receiving unit 2, an AD (analog-digital) conversion unit 3, and an analog demodulation unit 10, while the digital broadcast wave is converted into an audio signal through the antenna 1, the receiving unit 2, the AD conversion unit 3, a digital demodulation unit 20, and a decoding unit (decoder) 25. Also, the audio signals of the analog broadcasting and the digital broadcasting (hereinafter referred to as an analog audio signal and a digital audio signal, respectively) have phase differences (time differences) adjusted in delay units 11 and 21, and have volume differences adjusted in volume adjusting units 13 and 23, and then, are inputted to a mixing processing unit 4. Moreover, a mixing ratio control unit 41 of the mixing processing unit 4 outputs a switching signal SW according to a reception state signal ADR indicating a reception state of the analog broadcast wave or the digital broadcast wave, and the mixing unit 42 switches between the analog audio signal and the digital audio signal according to the switching signal SW, to be outputted, but in a process of such switching, the both signals are mixed and a mixing ratio is changed according to the reception state and outputted, and thus, rapid change in the volume or quality is alleviated.
As mentioned above, after the phase difference or volume difference between the analog audio signal and the digital audio signal is adjusted, both signals are mixed at a predetermined ratio according to the reception state, as well as switched therebetween, to be outputted, so that the sense of discomfort at the switching is decreased and the rapid change in the volume or quality can be alleviated.
Here, there are illustrated in FIGS. 9 and 10 examples of a relationship between electric field intensity and quality of the broadcast wave in the analog broadcasting and the digital broadcasting, respectively. As illustrated in FIG. 9, the more intense the electric field of the analog broadcast wave becomes, the higher quality the analog audio signal has, while the weaker the electric field of the analog broadcast wave becomes, the lower quality the analog audio signal has. Also, as illustrated in FIG. 10, in the digital audio signal, if the digital broadcast wave has an electric field whose level is more intense than a predetermined intensity level Sth1 and an error rate of the signal demodulated from the digital broadcast wave is lower than a predetermined error level Rth, sound quality is maintained at a substantially constant high level, while if the digital broadcast wave has an electric field whose level is weaker than the intensity level Sth1 and the error rate is higher than the level Rth, the sound quality becomes so low level as to be hardly reproduced as a sound.
Moreover, there is illustrated in FIG. 11 an example of a change in the quality of the audio signal outputted from the mixing processing unit 4 if the broadcast wave becomes a weak electric field in the IBOC method radio broadcasting. Before a time t2 when the broadcast wave has sufficiently intense electric field, the mixing processing unit 4 mixes the analog audio signal and the digital audio signal at a ratio of 0% and 100%, respectively, that is, selects only the digital audio signal to be outputted. Also, at the time t2 when the broadcast wave becomes an electric field weaker than an intensity level Sth2 (>Sth1) and thereafter, the mixing processing unit 4 mixes the analog audio signal and the digital audio signal at a predetermined ratio, to be outputted. Moreover, after a time t1 when the broadcast wave becomes an electric field weaker than the intensity level Sth1, the mixing processing unit 4 mixes the analog audio signal and the digital audio signal at a ratio of 100% and 0%, respectively, that is, selects only the analog audio signal, to be outputted. As illustrated by a solid line in FIG. 11, for example, during a period from the time t2 to the time t1, the mixing processing unit 4 smoothly changes the ratio of mixing between the analog audio signal and the digital audio signal.
However, even if the mixing ratio is changed smoothly, the mixing processing unit 4, at the time t2, selects only the digital audio signal, which is kept at a substantially constant high quality, to be outputted, and at the time t1, selects only the analog audio signal to be outputted whose quality is lowered as the broadcast wave becomes a weaker electric field. Thus, the change in the quality of the audio signal outputted from the mixing processing unit 4 during a period from the time t2 to the time t1 becomes more rapid than the change in the quality of the analog audio signal indicated by a long broken line in FIG. 11, for example.
Therefore, a sense of discomfort might occur when switching between the analog audio signal and the digital audio signal.